1-Hydroxy-6-substituted pyridones are known biocides. Although these pyridones exhibit excellent biocidal activity, and can be used in soaps, shampoos, skin care medicaments, cosmetics, adhesives, coatings, elastomers, sealants, wood, plastics and paints, currently their use in commercial products is limited because the pyridones are more expensive to manufacture than otherwise might be desired.
Heretofore 1-hydroxy-6-substituted pyridones are prepared from 2,6-dichloropyridine-N-oxide. As disclosed in U.S. Pat. No. 5,424,435, the process includes the steps of: (a) reacting 2,6-dichloropyridine-N-oxide, a hydroxy compound containing between 1 and 20 carbon atoms, and a base, optionally in the presence of water or an organic solvent, at an elevated temperature to produce a corresponding 2-chloro-6-substituted-pyridine-N-oxide, and (b) reacting the 2-chloro-6-substituted-pyridine-N-oxide with additional base to produce the corresponding 1-hydroxy-6-substituted pyridone.
One disadvantage associated with the prior art process is the low yield. Illustratively, at example 4 of U.S. Pat. No. 5,424,435, the patentees described the synthesis of 1-hydroxy-6-octyloxypyridine-2-(1H)-one from 2,6-dichloropyridine-N-oxide and 1-octanol at a 1:1 molar ratio in the presence of sodium hydroxide. The yield for the reaction was only 48%. In addition, in order to obtain a product of acceptable purity, the 1-hydroxy-6-(octyloxy)pyridine-2-(1H)-one has to be purified by recrystallization from ethanol and hexane. Such purification is cumbersome in the industrial settings and adds cost to the manufacturing process.
Methods to prepare 2,6-dichloropyridine-N-oxide, the starting material for the 1-hydroxy-6-substituted-pyridine-2(1H)-one, are known. Illustratively, it can be prepared by reacting dichloropyridine with hydrogen peroxide in the presence of trifluoroacetic acid. See Robert J. Rousseau and Roland K. Robins, Journal of Heterocyclic Chemistry, vol. 2, 196-201, 1965; and G. Zhu et al., Journal of Functional Polymers, vol. 23, 115-120, 2010. However, the prior art process used large amounts of trifluoroacetic acid, which could not be completely distilled off and had to be neutralized thus wasting expensive trifluoroacetic acid and producing byproducts that are potentially hazardous to the environment. Further, the prior art oxidation process gave only moderate yields of 2,6-dichloropyridine-N-oxide.
Accordingly, there is a continuing need in the biocide manufacturing industry for a cost effective and environmentally friendly processes to prepare 1-hydroxy-6-substituted pyridones and 2,6-dichloropyridine-N-oxide. The present invention is believed to provide an answer to that need.